A. Technical Field
The invention is concerned with device uses of electroplated palladium-group alloys. Most noteworthy is the use of such materials for replacing gold in electrical contacts. Gold replacement is of increasing importance due to rising gold prices.
B. History
Palladium, as well as other members of the platinum group, have been used in alloyed or unalloyed form as electrical contact materials for some years. These noble metals and alloys are known to be inert to usual ambients, have requisite low contact resistance, and are or may become less expensive than gold and gold-based alloys.
It has been found that palladium alloyed with less expensive elements retain electrical properties to a sufficient degree to permit their use under many circumstances. Electrical contacts of 60% palladium-40% silver are in general use. Related compositions, some ternary or higher order, have been found satisfactory in contacts, generally of striking rather than sliding design.
While sliding contacts, for example, as used in the connector art, continue to be dominated by gold-cobalt and other hard gold alloys, studies designed to substitute palladium and other platinum-group based materials are under way. These studies have been encouraging, and it is likely that such substitution will gain increasing acceptance.
In general, palladium-containing alloys, have been used in electrical contacts in the form of wrought members. These members, sometimes in the form of small buttons, are affixed by riveting, crimping, welding, cladding, or other mechanical means. Although the literature abounds with references to electroplating techniques, it is not apparent that they have found industrial use for palladium-containing materials. Such materials apparently compete with gold-based electroplate only in the form of mechanically affixed contacts.
It is apparent that workers have been concerned for some years with plating of platinum-group materials. As an example, Brenner in Electrodeposition of Alloys: Principles and Practice reviews attempts to electroplate Ag-Pt alloys from a variety of bath compositions through 1960. He refers, in particular, to the work of Grahm et al, 35, Plating, 1217 (1948) in which the authors report on the use of a concentrated lithium chloride bath for the plating of Pt-containing silver alloys and comments that, in his opinion, the bath would be feasible for Pd-containing alloys as well. In later work, Andreeva et al, Protective Metallic and Oxide Coatings, Metal Corrosion and Electrochemistry, 1965, the authors report specifically on the plating of palladium alloys from an aqueous LiCl bath. Other bath compositions for plating of palladium-containing alloys include the EDTA reported by N. T. Kudryavtsev et al in 7(2) Z. Metallov, 206 (1971).
Nonacceptance of EDTA, particularly for silver-containing alloys of platinum-group elements, is probably due to difficulties reported by Kudryavtsev et al. The ammoniacal plating system appears to operate under conditions of silver diffusion limited current, which adversely affects the deposit morphology, thickness, and rate of growth. Nonuse of highly acid bath solutions, such as aqueous LiCl, is possibly due to violent substrate attack. Unprotected base metal substrates undergo vigorous displacement reactions which compete with and prevent uniform electroplating.